Generally, a hybrid vehicle such as a hybrid electric vehicle (HEV) or a plug-in hybrid electric vehicle (PHEV) includes an engine and a motor operated by a high voltage battery to assist an output torque of the engine. The engine and the motor are a power source.
According to a driving situation, the hybrid vehicle is classified into a full (hard) type hybrid vehicle capable of driving the engine or the motor and a mild (soft) type hybrid vehicle in which the motor only assists an engine.
The mild hybrid vehicle is driven on the basis of the engine power, and the motor assists the engine power but does not operate alone.
In order to support both electric vehicle (EV) driving and hybrid electric vehicle (HEV) driving, the full type hybrid vehicle had a problem of increase in production cost that is due to large changes in internal structure of the vehicle and increase of parts of the vehicle.
However, unlike the full type hybrid vehicle, the mild hybrid vehicle does not significantly change internal structure of the vehicle in order to support the electric vehicle driving and the hybrid electric vehicle driving, and it is technically easy to implement the electric vehicle driving and the hybrid electric vehicle driving in the mild hybrid vehicle. Thus, production cost can be reduced.
The mild hybrid vehicle can improve fuel economy by 15% compared to a conventional vehicle, and 2% of the improvement of the fuel economy is obtained by regenerative braking energy generated when a brake pedal is stepped on.
According to regenerative braking regulations, a regenerative braking system should compensate for a limited regenerative braking force by a braking force when the regenerative braking force is limited.
The full type hybrid vehicle compensates for the limited regenerative braking force as much as the braking force by increasing brake hydraulic pressure in a hydraulic pressure regenerative braking system by the limited regenerative braking force.
However, when the hydraulic regenerative braking system is additionally applied to the mild hybrid vehicle, there is a problem of cost increase. If a regenerative braking function is abandoned without applying the hydraulic regenerative braking system, there is a problem that reduces fuel efficiency improvement effect of 2%.
Accordingly, there is a need to reduce manufacturing cost by omitting the hydraulic regenerative braking system, and there is a need to improve fuel economy using the regenerative braking function by satisfying the regenerative braking regulations.
The above information disclosed in this section is merely for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.